Phonon-polaritonic resonances on nanopillars of hexagonal boron nitride for surface-enhanced infrared absorption

Phonon polaritons (PhPs) in hexagonal boron nitride enable sharp midinfrared optical resonance with strong spatial confinement, making them promising for surface-enhanced infrared absorption (SEIRA) spectroscopy. Here, using colloidal nanosphere lithography, hBN nanopillar antennas are fabricated and their PhP resonances in a cost-effective way are demonstrated. By varying the diameters of the hBN nanopillars, the PhP resonance can be readily tuned to match the molecular vibrations of CBP (4,4'-bis(N-carbazolyl)-1,1'-biphenyl) molecules. Upon frequency matching, the coupling between the PhP resonance and the molecular vibration shows pronounced mode splitting, illustrating the SEIRA behavior with a coupling strength approaching the strong coupling regime. However, with slight frequency mismatching around 10 cm${}^{-1}$, the coupling strength decreases significantly, indicating a high sensitivity of the SEIRA activities to the resonance frequency of hBN nanopillar antennas. The findings provide a new method for the fabrication of PhP nanoantennas and may promote the development of PhPs in SEIRA-based midinfrared sensing applications. (© 2024 Wiley‐VCH GmbH)